Appropriate exception for signal handling failure

Raise an appropriate exception, mapped to a 400 error for signals
to resources which cannot handle signals.  In this case, the request
is inherently impossible to satisfy, because the plugin doesn't
support signals, so we should alert the user to this by saying it's
a bad request (rather than the current response which is a 500 error)

Change-Id: I8e47ee94e15309f378f62f2a9f6534a4eeddb389
Closes-Bug: #1346849
Merge "Appropriate exception for signal handling failure"